Free cutting alloys



Patented Jan. 7, 1936 I UNITED STATES PATENT- OFFICE,

2,026,545 FREE CUTTING ALLOYS 7 Louis W. Kempf and Walter A. DeanjCleveland, Ohio, assignors to Aluminum Company of America, Pittsburgh, Pa.,a corporation of Pennsylvania No Drawing.

3 Claims.

This invention relates to aluminum base alloys and it is especially concerned with those alloys containing a substantial amount of zinc.

' Aluminum base alloyscontaining zinc as a major alloying component have been widely used in both cast and wrought form because of their good casting and working qualities combined with satisfactoryphysical properties such as strength and ductility. Certain ,alloys having an aluminum-zinc base have been developed which have unusually high tensile and shear strengths on'the order-of 65,000 and 40,000'pounds per square inch, respectively. Thermal treatments aregenerally employed to improve the strength of the alloys and attain the high values indicated.

An alloy in which'such properties are attainable under suitable treatment is one containing from about 7 to 15 per cent of zinc, 0.5 to 2.5 per cent v While these operations have been performed on It is accordingly anobject of our invention to overcome these obstacles to easy and economical machining of alloys of the type disclosed above.

Another object is to accomplish the foregoing purpose without detrimental effect upon-the casting, working, and physical properties or susceptibility to thermal treatments.

. Our invention is predicated upon the discovery that the machining quality of the aluminum b'ase alloys containing from about 7 to 15'per cent of zinc, 0.5 to 2.5 per cent of copper, 0.2 to 2.5-percent of. magnesium, and 0.11:0. 1.5 per cent of manganese, may be markedly improved by the addition thereto of at least one of the elements A plication September 18, 1933, Serial N0. 689,883 7 lead, thallium, and bismuth. These elements are advantageously added in amountsof from about .0.1 to 6 per cent either separately or in combination: Alloys so formed possess excellent machin- -ing characteristics which may be described as being free machining or free cutting. Al-

loys of this nature may be readily cut on automatic machines at higher speeds than other alloys while. still retaining dimensional accuracy l and a satisfactory machined surface. For the purpose of our invention the elements lead, thallium, and bismuth are designated free machining constituents or elements and are regarded as being substantially equivalent by reason of the similarity in effect upon the machining quality of the alloys herein disclosed.

While lead, bismuth, and thallium may be used in amounts of from about 0.1 to 6 per cent-as set forth hereinabove, we prefer to use between about 1 and 4per cent .of these free machining constituents in alloys of the class described. The advantages gained from the use of the above named elements are especially evident in alloys containing from about 8 to 12 per cent of zinc, from about 0.5 to 1.5 per cent of copper, from about 0.5 to 1.5 per cent of magnesium, and 0.5 to 1.0 per cent of manganese, the balance being substantially all aluminum. As an example of the improvement in machinability gained through the use of free machining constituents the case of an alloy may be cited where the alloy was composed of about 9.5- percent of zinc, 0.8 per cent copper, 0.5 per cent magnesium, and 0.8 per cent, manganese, the balance being aluminum. By the addition of about 4 per cent of lead a smooth bright regularly cut machined. surface was obtained on the article when machined. The chips were short and easily broken so that dangerfof fouling the cutting tool was eliminated as compared to the long spirals obtained from the alloy containing no free machining constituent. The tensile properties of the alloy were not materially affected by the presence of lead. The elements bismuth and thallium produce an improvement in machining quality similar to that obtained through use of lead.

It has also'been found that the elements lead, bismuth, and thallium may be-advantageously used in combination .as well as separately to confer free machining characteristics on alloys of the type herein disclosed. Whenro employed together, the total amount of the added elements should not exceed about 8 per cent and it is cent. A surprising fact discovered concerning preferred to keep the total below about 6 per of lead and bismuth.

The tensile properties of the alloys are not materially altered by the addition of the free machining constituents in amounts up to about 4 per cent. Beyond this amount there is a decline in strength but this is accompanied by a somewhat better machinability oi the alloy because of the increased amount of iree machining constituent. The amount of lead, thallium 'and/or bismuth to be chosen for a given alloy is dependent upon the strength and degree 01 ma-w chinability required. I

The free machining constituents are most co'nveniently added in solid elemental form to the molten aluminum alloy since they melt at a temperature considerably below that ordinarily encountered in aluminum alloy melting practice. If more than about 1.5 per cent of these constituents is to be added to the alloy, the temperature of the molten bath should be raised above that commonly employed, the desired metals added, and the molten mass vigorously stirred to insure a thorough mixing the metals. The method here referred to or adding heavy low melting point metals to aluminum is more hilly described in copending application Serial No. 689,- 885 filed September 18, 1933. Alloys disclosed but not claimed herein are claimed in our 00-- pending applications, Serial Nos. 19,624 and 19,-

625, filed May 3rd, 1935.

The term aluminum" used herein and in the appended claims embraces the usual impurities i'ou'nd in aluminum ingot of commercial grade, or picked up in the course of the usual handling operations incident to ordinary melting practice.

The alloys herein disclosed may be subjected to the usual thermal treatments familiar to those skilled in the art for the purpose oi improving or altering their physical characteristics.

We claim: 4

1. An aluminum base alloy containing about 9.5 per cent of zinc, 0.8 per cent of copper, 0.5 per cent of magnesium, 0.8 per cent of manganese, and,;4 per cent of lead; the balance being aluminum.

2. An aluminum base alloy containing from about 7 to 15 per cent of zinc, from about 0.5 to 2.5 per cent of copper, from about 0.2 to 2.5 per cent 0! magnesium, from about 0.1 to 1.5 per cent oi.manganese, and from about 0.1 to 6 per cent of lead, the balance being aluminum.

3. An aluminum base alloy containing from about '7 to 15 per cent of zinc, from about 0.5 to 2.5 per cent oi. copper, from about 0.2 to 2.5 per cent of magnesium, from about 0.1 to 1.5 per cent of manganese, and from about 1 to 4 per cent of lead, the balance being aluminum.

LOUIS W. KEMPF. 

